In a previous article, we saw gravity’s role in shaping the large-scale structure of our universe and in bringing matter together to form life-essential stars and planets. Now, let’s explore the foundational role of another one of the four fundamental forces of nature, the strong force. Its contribution to making essential components of our universe and allowing us to exist is seen in how it holds nuclei together.1 Without the action of the strong force between nuclei (protons and neutrons), the periodic table would be limited to only the simplest element, hydrogen, and the universe would be devoid of life.
Hydrogen, as an atom, consists of just a single proton for its nucleus, surrounded by an electron held in orbit by the electric force (which we’ll focus on in a subsequent article). The aspect of the electric force that causes particles of like charge, such as protons, to experience a repulsive force that grows stronger with proximity, calls for another force to hold together every element with more than one proton.
Exceptionally Small Distances
Of necessity, the strong force must overpower the repulsive electric force between protons at distances relevant to the atomic nucleus, or at approximately 10-15 m. At these exceptionally small distances (about one hundred thousand times smaller than the overall size of an atom), the electric repulsive force between just two protons becomes surprisingly large — on the order of 50 pounds of force! So, we see why the nuclear force holding them together is called the “strong” force!
Here’s another amazing consequence of the incredibly powerful nature of the strong force holding the nuclei of atoms together — the energy stored in the nucleus, like the potential energy of a tightly compressed spring, adds up to more than we might imagine. For example, in a typical gold ring of five grams mass, the total energy involved in holding all the gold nuclei together (about 1.5×1022 atoms of gold make up the ring) adds up to nearly 522,000 kWh of energy — enough to supply all the electrical energy of an average household for 49,000 years! Another way of looking at it is if the strong force suddenly turned off, the nuclei in an object as small as a gold ring would explode with energy equivalent to a small atomic bomb.
The strong force is about 100 times stronger than the electric repulsive force for protons in close proximity, as in an atomic nucleus. Also unusual is the extremely short range of the strong force. For it to engage, nucleons (both protons and neutrons, upon which it acts equally) need to almost touch one another. The existence of all the elements that contribute to material things, and therefore our very lives, depends upon the nature of the strong force in relation to other fundamental forces of nature.
Purely a Nuclear Force
The strong force is purely a nuclear force; beyond its key role in forming the various elements of matter, it exerts no influence in chemical reactions due to its exponentially short range. Compared to the size of the nucleus, the outer range of the orbits of electrons around an atom is about one hundred thousand times farther away, giving also the approximate distance between nuclei in a molecule. If the range of the strong force varied with distance like it does for gravity and the electric force (growing weaker with the square of the distance), the attraction between nuclei of different atoms would overwhelm chemical interactions, and again, life would be non-existent.
To further appreciate the life-essential nature of the strong force, consider its role in producing the energy that allows stars to shine. Deep in the core of a star like our sun, gravity compresses the predominantly hydrogen gas composing stars into a supremely dense, hot fluid, about 14 times denser than lead. At a temperature of 15 million degrees Celsius, the thermal energy of hydrogen nuclei in the sun’s core causes collisions in which their proximity allows the strong force to capture and bind them into a heavier nucleus of helium. (Some details are left out here, for the sake of brevity!)
The Sun on Your Face
An amazing thing happens in this process: when bound by the strong force, the four nucleons that make up the helium nucleus (two protons and two neutrons) end up with a combined mass that is slightly less than the mass of the same four nucleons when separated. This mass loss is equivalent to the nuclear binding energy produced by the strong force. The net result is a release of energy with the formation of every helium nucleus via fusion. Next time you feel the warmth of the sun on your face, consider that it comes primarily from the conversion of matter into energy, mediated by the strong force fusing hydrogen into helium within the core of the sun.
As a final manifestation of the strong force, let’s take a look at a denizen of the astronomical realm not discovered until about 60 years ago — neutron stars. These objects are actually stellar remnants, the leftovers from massive core-collapse supernovae. Near the end of the life cycle of a massive star, fusion stalls at the production of iron, which has the highest binding energy per nucleon of any element. This simply means that the star cannot produce more energy by fusing additional nucleons onto an iron nucleus — the star has run out of gas.
Gravitational collapse ensues in dramatic fashion, compressing the core so powerfully that electrons and protons are forced to combine into neutrons, which being neutral of charge, can be compressed greatly into a small sphere of neutrons of exceptional density. The rest of the old, massive star is blown into space during the supernova explosion, and the core is exposed as a hot, ultra-dense ball of neutrons.
These neutron stars typically have a radius of only 10 km, and yet have more mass than our entire sun (with its radius of 700,000 km). The density of a neutron star is so great that a sugar cube sized lump of it would have a mass of a billion metric tons! Since it’s composed of neutrons, one could think of it as an enormous nucleus out in space, but with one important difference. Atomic nuclei are held together by the strong force, but neutron stars are held together by gravity.
Intelligently Designed for Life
The strong force still plays an important role in neutron stars in an unexpected manner — under these extreme conditions it serves as a repulsive force between the neutrons, hindering gravity from crushing it into a black hole. The attractive nature of the strong force between any two nucleons draws them towards an equilibrium separation distance of about 0.7×10-15 m (0.7 fm), but the same force becomes repulsive if the nucleons are squeezed closer than about 0.5 fm.2 This odd feature of the strong force maintains a minimum size of atomic nuclei and helps to hold off complete gravitational collapse of neutron stars.
The strength of the “repulsive core” of the strong force has a limit, and if gravity becomes strong enough to exceed it, the neutron star will collapse into a black hole. Theoretical astrophysics predicts this limit to occur for neutron stars more massive than about twice the mass of our sun, which is consistent with the mass estimates of nearly all neutron stars that have been discovered. A change in the strength of the repulsive core of the strong force would have immediate effects on the number of stellar-mass black holes in our galaxy. This and other consequences relating to neutron star properties may show that even the obscure details of the strong force have been intelligently designed for life.
Notes
- In the field of particle physics, the strong force primarily binds together the quarks composing protons and neutrons (and other baryons). The inter-nucleon force is regarded as a residual effect of the strong force. (https://www.energy.gov/science/doe-explainsthe-strong-force )
- E. R. Hedin, “A higher-dimensional model of the nucleon-nucleon central potential,” Frontiers of Physics, 9(2), 234-239 (2014), DOI 10.1007/s11467-013-0393-x.